The present invention relates in general to drive systems for electric vehicles, and, more specifically, to circuitry for combining the functions of precharging of a capacitor upon energizing of the electric drive and discharging the capacitor upon deactivation of the electric drive.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), utilize inverter-driven electric machines to provide traction torque. A typical electric drive system may include a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches (i.e., relays) to an input capacitor for buffering the battery voltage. A DC-DC converter (also known as a variable voltage converter, or VVC) couples the input capacitor to a main DC linking capacitor that supports a high voltage DC bus. The VVC may bi-directionally direct a current flow between the input capacitor and the linking capacitor to regulate a voltage across one of the capacitors. A three-phase motor inverter is connected between the main buses with outputs of the inverter connected to a traction motor in order to convert DC bus power to an AC voltage coupled to the windings of a traction motor in order to propel the vehicle. During deceleration of the vehicle, the motor can be driven by the vehicle wheels and used to deliver electrical power to charge the battery during regenerative braking of the vehicle, with the DC-DC converter working in the opposite direction to convert the generated power to a DC voltage appropriate for charging the battery pack. In some vehicles, a generator driven by an internal combustion (gasoline) engine is provided to generate electric power to charge the battery. A second three-phase inverter typically connects the generator output to the high voltage DC bus.
Due to the high voltages present when the electric drive is in use, special precautions are necessary during activation and deactivation of the drive. During activation, for example, the contactors are opened at a time when the capacitors are discharged at about zero Volts. Closing the contactors with the capacitors in a discharged or low charged state would present a low impedance to the battery pack, resulting in a very high inrush current that could cause damage to the contactors and other components. One solution is to provide a constant resistance between a contactor and the capacitors. However, use of a current-limiting resistor in series with the contactors is undesirable after the initial precharging because of the associated voltage drop and power consumption it would cause during subsequent normal operation. Therefore, a separate circuit branch, or precharging circuit, is often used. The known precharging circuits utilize a switch and a resistor in series between the DC supply and the capacitors. Turning on the switch allows the capacitors to be charged through the resistor, and the presence of the resistor limits the inrush current to prevent damage to the switch. Once the capacitors are precharged, then i) the main contactors can be closed without receiving any inrush current and ii) the precharge switch can be opened so that the precharge resistor is disconnected.
During deactivation, it becomes necessary to discharge the capacitors. A shutdown of the electric drive system can result from a vehicle key-off, a high-voltage DC interlock fault, or a vehicle crash, for example. During shutdown, the battery is quickly isolated from the rest of the electric system by opening the mechanical contactors. This also isolates the electric charges present on the DC capacitors. Due to safety requirements, the HV capacitor charges should be quickly discharged within a specific time. For example, U.S. Federal Motor Vehicle Safety Standards (FMVSS) may require that the voltage on the DC link capacitor must be less than 60V within 5 seconds in certain circumstances.
The simplest conventional methods for discharging the link capacitor use a resistance placed across the capacitor to dissipate the charge. The resistor placement can be passive or active. A passive discharge resistor (PDR) is hard-wired in parallel with the link capacitor. The passive resistor must have a relatively large resistance to avoid excessive power loss during normal operation. Consequently, it could take one to two minutes to dissipate an HV charge down to a safe level. To discharge more quickly, an active discharge circuit uses a resistor in series with a transistor switch so that the charge can be selectably dissipated through a smaller resistance value.
The circuit components for the active discharge circuits and at least some components for a precharge circuit are typically included on a printed circuit board in an Inverter System Controller (ISC) module. Thus, the size, component count, and cost of an ISC module are all increased. It would be desirable to perform the precharge and discharge functions with fewer components so that size and cost of an ISC module can be reduced.